This invention relates in general to active noise cancellation systems, and in particular to such systems which employ a microphone proximate a listener's ear to receive essentially the same ambient noise received by the ear and a speaker for producing sound in the vicinity of the microphone to cancel the ambient noise.
Prior art as in U.S. Pat. No. 2,972,018 by Hawley et al. (1953) provide for a microphone which responds to ambient noise and a speaker which transmits an altered version of the ambient noise to provide a measure of noise cancellation in the region of the microphone. Virtually all subsequent patents of improvement incorporate some variant of this concept, but they all use microphones.
This invention takes advantage of the known fact that certain kinds of speakers (e.g. diaphragm speakers) respond to pressure waves of incoming sound (e.g. ambient noise) and produce an output signal representative of the incoming sound.
Other advantages and attributes of this invention will be readily discernable from a reading of the text hereinafter.